I don't think there is a single answer. Instead, I would approach it as this: ifIf a plugin-question is primarily about PyQGIS, spatial analysis, some aspect of making a map, or spatial data formats then it belongs on GIS.SE SE.
If the issue is more strictly related to non-spatial programming then StackOverflowStack Overflow is more appropriate.
I would think the examples you listed would usually fall on the GIS.SE SE side, except for things like "having problems with memory management in Python plugin I'm developing for QGIS".